Mobile application development just got more interesting. Rebtel, a voice over IP, or VoIP, solution that competes with Skype, has entered the industry and has announced the private beta of its voice platform for iOS and Android mobile devices. With the addition of this SDK, developers can now have calls be made right through their own applications, often for free or at a cheap rate.

Rebtel’s Voice Platform offers developers a strong tool to help scale their applications — and the company claims it’ll only take 15 minutes for the features to be integrated.

Once added, Rebtel acts like a layer in-between the application and the user that’s making the call — it will know how to direct the call: if it’s to another user using the same Rebtel-enabled app on a different device, then the call will be free.

However, if it’s an international call, then the service will process the call, resorting in the cost being 95% cheaper than if it had gone through an operator.

Andreas Bernstrom, Rebtel’s CEO, says:

Voice is a feature that can significantly augment the user experience in a variety of verticals and in the long term, we see the Rebtel Voice Platform as a game-changing tool for developers. With the launch of this free service, thousands of content publishers and smartphone developers will have the opportunity to enrich their applications with a voice layer thereby increasing usage and appeal, or they can even distribute their own branded mobile VoIP app without any large back-end commitment.

The launch of this private beta is taking place with the cooperation of three communications partners: VIVfone, MobisleApps, and Voxy. Rebtel believes that this platform is happening at a critical time for developers looking to find new ways to monetize their user base, while also finding solutions to make it more social. For gaming apps like poker and social networks that share photos, having the ability to make free app-to-app calls is probably a good idea.

However, let’s not forget that there are already services out there helping developers with voice calls, of which Skype is likely a developer might turn to. In addition, there’s also Twilio, Tropo, Google Voice, TextPlus, and perhaps dozens more. What makes Rebtel so special?

It claims that it’s free and simple and that’s worthy of a differentiator. With respect to Twilio, Bernstrom says that the company charges for everything related to the call — for a developer, this could be a deal-breaker. With TextPlus, the service doesn’t have a development side to it that allows others to build off of it. And there’s always WebRTC, but you need to be a “strong coder”. Bernstrom believes Rebtel’s solution is something that developers will want to use.

There is currently no cost to use the voice platform for either the developer and user, but Rebtel believes that should hundreds of thousands of calls be sent each day, the company will most likely result in a freemium model in order for the company to recoup some of the costs.

There are more than 17 million users on Rebtel’s network with a run rate of over 1 billion minutes per year. It is forecasted to reach $80 million in revenue by the end of 2012.